The present invention relates to a synthetic cornea.
More precisely, the invention relates to an optical prosthesis which can be put into place in the cornea after a central opening has been formed therein when the cornea no longer performs its transparency function.
Accompanying FIG. 1 shows the central portion of the cornea 10 of an eye. It is made up of a plurality of membranes or lamellae placed one on another. Starting from the posterior face, i.e. the inside face 12 of the cornea, there can be seen the endothelium 14 which is constituted by a layer of cells that do not regenerate, Descemet""s membrane 16, intermediate lamellae 18 forming the stroma of the cornea, and then Bowman""s membrane 20, and finally the epithelium 22 which constitutes the anterior face 24 of the cornea. The epithelium is constituted by a plurality of layers of cells with great propensity towards regeneration. The total thickness of the cornea, in its central region, is about 500 microns (xcexcm) and together the stroma plus the epithelium constitute 97% of the total thickness.
In known techniques for putting a keratoprosthesis into place, an incision is made in the form of a cylindrical passage 26 passing right through the thickness of the cornea. In the most widely known type of keratoprosthesis, known as intrastromal keratoprosthesis, the prosthesis is constituted by a cylindrical optical piece placed in the passage 26, and it has an annular skirt which is inserted between the lamellae 18 constituting the stroma of the cornea in order to hold the keratoprosthesis mechanically in place in the cornea. That interstromal technique of keratoprosthesis is known to give results that are poor or even clearly bad.
That is why proposals have been made in particular in patent application WO 97/27824 in the name of the Applicant, for another form of keratoprosthesis in which the cylindrical optical portion is provided with a posterior lateral skirt that is pressed against the inside face of the cornea in order to hold the keratoprosthesis mechanically in the cornea. That solution gives results that are much more satisfactory.
Whatever the way in which the keratoprosthesis is held in the cornea, prior techniques have required a passage to be made in the central portion of the cornea passing right through the cornea. The fact of making such a smooth opening presents various drawbacks, in particular concerning risks of leakage or infection.
When the cornea has been damaged in severe manner (burns, etc.), keratoprostheses with post-corneal bearing surfaces constitute the best solution.
In more numerous cases, the damage to the cornea is restricted and Descemet""s membrane is not affected. A corneal graft is then possible. However, problems associated with viral contamination and with the shortage of tissue for grafting, mean that it would be advantageous to have a synthetic cornea suitable for being put into place in the cornea while still enabling Descemet""s membrane to be conserved together with the endothelial layer so as to avoid the above-mentioned complications due to an opening passing through the cornea.
Document U.S. Pat. No. 4,842,599 describes a synthetic cornea which can be put into place in the cornea while conserving Descemet""s membrane. However, in order to hold the synthetic cornea mechanically in place, it is provided with an annular fixing skirt, said skirt having orifices for receiving sutures. Such a solution is not satisfactory since it gives rise to a large amount of trauma over an extended zone of the cornea.
An object of the present invention is to provide a synthetic cornea suitable for putting into place in the cornea and for being held effectively therein while avoiding perforating the cornea right through, whenever the state of the posterior planes of the patient""s cornea make such a solution possible.
According to the invention, this object is achieved by a synthetic cornea for placing in a recess formed in the cornea of the eye, said recess opening out into the anterior face of the cornea and not into its posterior face so as to leave Descemet""s membrane intact, the synthetic cornea being characterized in that it comprises a piece of flexible transparent material having an axis of circular symmetry presenting an anterior face in the form of a spherical cap of radius Ra and a posterior face in the form of a spherical cap of radius Rp, the two faces being spaced apart in their central zones by a distance e, and a side wall substantially in the form of a truncated cone having its axis coinciding with the axis of symmetry and having its apex disposed in front of said anterior face, the angle a between the section of the side wall and the section of the posterior face in a section plane containing said axis of symmetry lying in the range 10xc2x0 to 35xc2x0.
It will be understood that because the prosthesis is generally in the shape of a truncated cone, it can be received in a recess in the central portion of the cornea that is of similar shape without it being necessary to perforate Descemet""s membrane and the endothelium. Nevertheless, because of its frustoconical shape, the synthetic cornea is held effectively in the cornea, pressed against Descemet""s membrane. When the angle a is greater than 35xc2x0, the risk of expulsion becomes great.
The angle a is preferably about 25xc2x0 to 30xc2x0, thus providing still better retention of the prosthesis in the cornea.
More preferably, the radius of curvature of the anterior face of the transparent piece is as close as possible to that of the anterior face of the cornea, i.e. it lies in the range 6.5 millimeters (mm) to 8.5 mm. This avoids any risk of disturbing movements of the eyelids over the cornea, which would be likely to interfere mechanically with retention.
Another problem which arises when placing a synthetic cornea in the natural cornea, is the risk of cell proliferation that can develop on the posterior face of the synthetic cornea, causing said posterior face to become progressively opaque by forming a retroprosthetic membrane.
In an improved embodiment of the invention, the synthetic cornea further comprises an annular skirt connected to the side face of the piece forming the synthetic cornea, the posterior face of said skirt being placed on the same spherical cap as the posterior face of the synthetic cornea proper, said skirt having a thickness of not more than 100 xcexcm.
The skirt serves as a barrier against cell proliferation towards the posterior face of the synthetic cornea proper. It has no mechanical function of holding the synthetic cornea in the natural cornea. That is why it can be of very small thickness, less than 100 xcexcm and preferably less than 50 xcexcm. Since the Young""s modulus used is preferably less than 1 megapascal (MPa), it will be understood that the skirt has no significant mechanical strength.
In order to further improve the barrier effect against cell proliferation, provision can be made for the edge of the skirt to be xe2x80x9csquare-cutxe2x80x9d.
Other characteristics and advantages of the invention appear more clearly on reading the following description of various embodiments of the invention given as non-limiting examples.